Light-emitting diodes (LED) or laser diodes (LD) have been developed, which emit lights ranging from a blue to red visible range to violet or ultraviolet. A display device using such multicolor LEDs in combination is used for displays or traffic signals. Further, a light-emitting device has also been proposed, in which the luminescent color of a LED or an LD is changed by a phosphor. For example, JP-B-S49-1221 discloses a method wherein a laser beam which emits a radiation beam having a wavelength of from 300 to 530 nm, is applied to a phosphorescent material (Ln3−x−yCexGdyM5-zGazO12 (Ln represents Y, Lu or La, M represents Al, Al—In or Al—Sc, x is from 0.001 to 0.15, y is at most 2.999, and z is at most 3.0) to let it emit a light to form a display.
Further, in recent years, as an emission source for an image display device or an illumination device, a light-emitting device for white emission is proposed, which is constituted by a combination of a gallium nitride (GaN) type LED or LD having high luminous efficiency and having attracted attention as a semiconductor emission element of a blue light and a phosphor as a wavelength conversion material. JP-A-10-190066 discloses a white light-emitting device wherein a blue LED or a LD chip made of a nitride type semiconductor is used in combination with a phosphor having a part of Y in a cerium-activated yttrium.aluminum.garnet phosphor substituted by Lu, Sc, Gd or La, so that the blue light and a yellow light emitted from the phosphor are color-mixed to form white emission. JP-A-10-247750 discloses a color-conversion mold member, a LED lamp, etc. obtained by a combination of yttrium.aluminum oxide fluorescent materials activated with cerium, which have at least one element component selected from the group consisting of Ba, Sr, Mg, Ca and Zn, and/or a Si element component.
Further, JP-A-10-242513 discloses a phosphor having a part of Y in a cerium-activated yttrium.aluminum.garnet phosphor substituted by Sm. Further, JP-A-2003-505582 or JP-A-2003-505583 discloses the effects of a phosphor having Tb added to a cerium-activated yttrium.aluminum.garnet phosphor.
However, in such a combination of the cerium-activated yttrium.aluminum.garnet phosphor and a blue LED or a blue laser as disclosed above, the emission intensity of white is still insufficient, and therefore, improvement in the efficiency of the blue LED has been required. However, also with regard to such a phosphor, the emission intensity is still insufficient, and therefore, further improvement in the efficiency has been required to realize energy-saving illumination.
Further, with regard to the temperature characteristics, when the LED or the LD as the first illuminant turns on, the ambient temperature of the chip increases, whereby the efficiency of the LED or the LD tends to decrease, and also the phosphor contained in the second illuminant, tends to lose its luminance substantially due to such temperature increase. Usually, the qualities change according to the composition of the host material, and the type and the amount of the activator, and therefore, it is required to employ a material having high emission intensity and a small decrease in the emission intensity due to such temperature increase.
With regard to the afterglow characteristics, especially in a case where the LED or the LD as the first illuminant is used for a display or a back light by means of pulse driving, if the afterglow time of the phosphor contained in the second illuminant is extremely short, there will be a problem such that flickering results and no adequate image characteristics can be obtained, and thus, the improvement is required.